HIGH-SPEED OCEAN CABLE TELEGRAPHY 263 



An obvious great difficulty with coil-loaded deep-sea cables lies in 

 the mechanical problem of laying a cable to which coils are attached 

 or in which coils are inserted in a way to give a mechanical irregularity. 

 Unless the coils could be made extremely small their presence would 

 certainly interfere with passing the cable smoothly through the paying- 

 out machinery. Cable laying and repairing are sufficiently difficult 

 and hazardous under the most favorable conditions and any alteration 

 in cable structure which would make these tasks more difficult is 

 certainly to be avoided if possible. Permalloy cores for loading coils 

 might, however, to some degree eliminate this objection to coil- 

 loading, since with a permalloy core the loading coils may be made 

 smaller with the result that less difficulty would be caused by the 

 increased size of the cable at the points where the coils were inserted. 



The problems of maintaining good insulation and sound joints at 

 the loading coils are probably much more serious. Conductor joints 

 in a cable are frequently subject to considerable stress and even with 

 the relatively simple joints required for ordinary deep-sea cables 

 trouble is occasionally experienced. With loading coils inserted in 

 the cable both the coils and the joints between the coils and the core 

 must be subject to great stress, and since the coils must be many in 

 number to be effective, the probability of faults with even the best 

 imaginable construction would be greatly increased. 



From the electrical point of view an apparent advantage of coil- 

 loading is that it might conceivably permit adding the required 

 inductance without introducing so much a.c resistance and thereby 

 permit more closely approximating the ideal loaded cable. On the 

 other hand, coil-loading has an electrical disadvantage which has 

 not generally been appreciated but which is of serious practical 

 consequence. This disadvantage lies in the distortion of signal-shape 

 arising from the lumped character of the line. With uniform con- 

 tinuous loading the line is electrically smooth; such a line may 

 introduce distortion but this distortion can be compensated for by 

 terminal apparatus. With coil-loading the line is, in effect, a net- 

 work of as many sections as there are loading coils. Such a line 

 introduces a new type of distortion which arises in the so-called 

 filter oscillations. Although it is theoretically possible to compensate 

 for this effect by terminal networks, the circuits required are extremely 

 complex and practically the limit of speed is set by the frequency 

 of signal impulses at which filter oscillations begin to cause serious 

 distortion. This effect can be practically eliminated by making the 

 distances between coils sufficiently small, but as the distance between 

 coils is diminished the otherwise possible advantages of coil-loading 

 are likewise diminished. 



